Plain Language Summary

Disclaimer: the following information is not medical advice, please consult your doctor before making drastic lifestyle changes. This is very important if you have diseases that can be modified by ketosis, or if you’re on medication: “patients on diabetes medication who use [a ketogenic diet] should be under close medical supervision” - Yancy et al., 2005

The basics of keto

With the ketogenic diet, you aim to eat 20 to 70g of carbohydrates per day. The body then starts using fat and ketones as primary energy sources. A high protein diet (i.e. 2.2 g/kg) does not seem to prevent ketosis (read more). Some claim that keto is the best diet for improving body composition, endurance, and strength. We have reviewed the ketogenic literature and come to the following conclusions:

Note: By design, this table presents a simplified summary of the keto research. Continue reading to learn more!

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Keto does not seem to offer a distinct advantage for fat loss when both groups in a study eat the same amount of calories. People on the ketogenic diet typically lose body weightquicker, but this is most likely due to glycogen depletion and water loss.

The most commonly measured ketone body, Beta-Hydroxybutyrate (BHB), does not seem to correlate with fat loss. However, this depends on which studies we include and exclude from our analysis. Go to ketone levels and fat loss for graphs, details, and discussion.

We generally see greater lean body mass (LBM) loss in ketogenic diet groups. Note that lean body mass contains water, glycogen, and muscle protein, by definition. It is hard to say with certainty that LBM loss implies greater “dry” muscle protein loss. “Wet” LBM can come and go quickly because it consists of water and glycogen.

Most studies suggest that aerobic and anaerobic performance is either impaired or maintained on a ketogenic diet. However, many of the studies have small sample sizes, no control group, or are quite short (2-6 weeks). These limitations, and many others, reduce our ability to draw strong conclusions.

Control groups tend to gain more strength than keto, yet most studies find no statistically significant differences between groups. In most studies, the keto group ate more protein than control. We can’t rightfully compare them without matching protein and total energy. Several of the studies do not measure ketosis.

The data we have discussed in this article deals with how people respond on average. However, individuals can respond very differently to the same diet. Some have a lot of success on keto, and some do not. Hence, one diet might not work for you, but perhaps another will.

The graph below shows what happens before and after 5 people were put on a ketogenic diet. Their endurance, as measured by time to exhaustion, varied from person to person (Phinney et al., 1983).

Graph by Brad Dieter and Adam Tzur

Issues in keto research

The keto literature has many issues, conflicts of interest, and limitations.

Here are some examples:

Many studies measure blood ketone levels, yet several do not. In the latter studies, we do not know if the participants were in a ketogenic state or not.

Several studies do not directly track how much the study participants ate (i.e. they rely on self-reported food questionnaires). This can be somewhat of a limitation, especially in studies where they didn't measure ketone levels. 1While dietary recalls or food-frequency questionnaires aren't perfect, they are necessary when subjects aren't constantly being monitored (such as in a metabolic ward or hospital setting)

Keto groups tend to eat more protein and less calories than control groups.

Many studies have poor methodology or experimental design. 2Many studies also have solid methodologies. The limitations presented here do not imply that the entire keto literature should be rejected, as some have suggested.

(lack of) carbohydrates affect body composition estimations.

A complete keto limitations article is currently in the works. A link will be added later.

Other Publications

SCI-FIT is a transparent organization; all information is freely available. To request a correction or addition to this article, please email adam@sci-fit.net. If your arguments and sources are valid, they might be added to the article with full credit. Feel free to send general feedback as well. All comments, praises, or criticisms are welcome, no matter who you are or where you stand in this discussion.~Adam

Yet, the 50 g carb “limit” might vary from person to person (Adam-Perrot et al., 2006). Hence, we can’t say that 50 g is a rule that applies to everyone. In fact, data indicate that the carb limit is higher. We will cover this in-depth in part 3 of this series. Subscribe to the newsletterto stay informed.

In terms of dietary protein and fat, the keto diet isn’t set in stone. It is usually moderate/high protein and high fat, but there are several variants of the ketogenic diet. The classic variant sometimes used to manage epilepsy is very high fat, low carb, and low protein (Roehl and Sewak, 2017).

Will a high protein intake prevent ketosis?

It has been said that eating a lot of protein will kick you out of ketosis because of gluconeogenesis2When protein is turned into glucose. In spite of this claim, protein intakes from 122 to 176 grams (1.1-2.2 g/kg) per day did not prevent ketosis in several studies.

Studies reporting protein and ketone levels

Critical thinking exercise

Which factors could influence a person’s degree of ketosis, excluding dietary fat, carbohydrates and protein?

A caloric deficit typically causes us to lose weight, but if we eat sufficient amounts of protein and do strength training, we can retain or increase muscle mass. This is why it’s very important to distinguish between weight loss and fat loss. Weight loss includes loss of fat mass andlean mass (i.e. water, glycogen, and muscle protein).

Challenging this model, some keto proponents argue that it is the degree of ketosis or insulin inhibition that leads to fat loss. According to this argument, the decrease in insulin and elevation in ketones is what leads the body to become fat adapted. The body should then burn more body fat compared to a calorie restricted diet. As such, keto may have a “metabolic advantage” (Volek et al., 2004).

To answer whether keto is superior for burning fat, we’ve compared both the CICO model and carb restriction model in the sections below.

How keto affects body fat and muscle mass

Overview of the literature

In this section, we’ve summarized the keto research on humans. Overall, the data follows predictable patterns. A caloric deficit lead to loss of fat and lean body mass, however lean mass was typically retained or increased when subjects did strength training and ate enough protein.

As shown in the tables below, many of the keto studies did not match protein 3Protein matching means that two groups in a study ate about the same amount of protein. A higher protein intake could improve satiety (Veldhorst et al., 2010; Johnstone et al., 2008)(and therefore adherence), and it also preserves LBM.

Calories are matched in some studies. In the case of improper matching, the group that ate less calories would most likely lose more weight.

This was not the case in Volek et al., 2002 and 2004. These studies are outliers/anomalies because they run counter to the rest of the literature. In both Volek studies, keto groups lost more fat than the alternative/control diet, even if the control diet ate less calories. Nonetheless, we don’t know if the control group was in a greater calorie deficit. Their baseline diets are not reported in the study.

The “baseline diet” refers to how much subjects ate habitually, i.e. before they started the study. For example in Meckling et al., 2004, both groups ate similar amounts of calories during the study:

Keto1534 kcal(33% ⇓)

Alt.1452 kcal(29% ⇓)

The percentages below the calories show how much less the groups ate compared to their habitual intake. Keto ate about ~80 kcal morethan the alternative diet, in terms of absolute values. Yet, keto ate lessthan alt, relative to their habitual pre-study diet. Their relative caloric deficit was 4% deeper than alt (33% vs 29% reduction).

Beware of interpreting these numbers without context.

Data in this table are taken from RCTs and cross-over studies in the fat and LBM comparison tables shown below. Single-arm studies are excluded.

Exclusions from general overview:

Studies comparing 2 keto groups are excluded

Studies that only report goal calories are excluded from “Studies with similar reported energy intakes between groups”-column.

N.S. = No statistically significant differences between groups (however there might have been significant differences from baseline)

Studies without exercise (11)

Data from the studies are averaged in the following table. Beware of interpreting the numbers without context:

Note: data in this table are averages based on data from RCTs and cross-over studies in the fat and LBM comparison table. Single-arm studies are excluded. If a range is given in a study (i.e. calories or protein intakes fluctuated over multiple weeks), the highest number of the range was chosen for the calculation.

A red background indicates that kcal or protein was not matched between the keto group and the alternative diet group (more than 15g difference in daily protein intake). Yellow indicates uncertainty due to lack of data reporting or methodological issues. For example, study design might call for calorie matching, but if they didn’t report food intake (i.e. via self-reporting), then we don’t know what they actually ate. Green indicates appropriate matching (less than 15g difference between groups).

Carb intake and ketones column

Red indicates that the subjects in the keto group were probably not in ketosis. Yellow indicates that they might be in ketosis, or that there is some uncertainty. Green indicates that they were in all likelihood in ketosis, as measured by dietary recall combined with blood, breath, or urine tests.

Fat loss and LBM columns

Green indicates that keto lost more fat or retained more muscle than the alternative diet (statistically significant between-group difference). Grey indicates no statistically significant differences between groups. Red indicates that keto was inferior in terms of fat loss or LBM maintenance/gain.

indicates that there are big limitations in the study

indicates that subjects did resistance training during the study, it does not imply that subjects were resistance trained

Alt. = Alternative diet group

x% ⇓ indicates how much participants’ caloric intake was reduced by, compared to their habitual intake. Look at Wood et al., for example. Both groups ate the same amount of calories, but keto reduced their intake more, relative to baseline.

Meirelles and Gomes, 2016: No objective measure of ketosis, keto group increased their carb intake every week (80g during last week before the test, confounder), 13 years age diff between groups, subjects chose their own diet group, etc.

Stimson et al., 2007: Improper data reporting. Reports a common/shared baseline for bodyweight in both groups, but it was a crossover study with 3 days washout between interventions. Impossible to extract data.

Merra et al., 2016: 25 kg body weight difference between groups at baseline. FM/FFM/BW numbers do not add up. Only within group significance tests were performed comparing baseline to post (paired t-tests). No between-group testing

Volek et al., 2002: Not randomized (subjects self-selected their groups, one control group with no dietary change)

Note: if a range is given (i.e. calories or protein intakes fluctuated over multiple weeks), the highest number of the range was chosen for the calculation.Agee, 2015 excluded from carb intake calculation due to massive weekly variation in carbs eaten.

Studies without control groups (single-arm trials) (4)

Strictly speaking, Wood et al., 2007 isn’t a single-arm trial because it compares two keto groups. But, we found it fitting to put it here because it doesn’t fit in the tables where keto is compared to a control group.

Buscemi et al., 2009: Subjects were not in keto by the end of the study because they increased carb intake weekly: “All subjects in the low carbohydrate diet group had positive urinary ketone test at T5 against none of the Mediterranean diet group; at T60, urinary ketones were found positive only in one subjects of the Atkins diet group.”

Yancy et al., 2004: Diet plan changed after week 10. They might have exited ketosis after a couple of weeks due to reintroduction of carbs. Poor reporting for ketonuria makes it difficult to determine how many subjects were in actual ketosis.

Critical thinking exercise

Why did we choose to analyze body fat and lean body mass instead of body weight?

Discussing how fat loss compared between groups

Three studies found that keto lost more fat mass than control when calories were matched. In all three of the studies, the participants were obese, and in two studies the participants were women (Brehm et al studies). It’s difficult to say exactly why the keto groups lost more fat, but here are some potential reasons:

Protein was not matched between groups. Since protein is a satiating macronutrient, it can help adherence to the diet. As such, lower protein intakes may lead people to eat more.

The Brehm studies asked keto participants to eat ad-libitum (as much food as desired). There may have been a positive psychological effect for this; it is easier to adhere when you don’t have to count calories because you only have to reduce carbs. The control groups, in comparison, had to count calories to hit a specific caloric goal. This could lead to underreporting.

There might be metabolic advantage to keto.

Carbohydrate depletion could affect how body fat levels are estimated in most tools that assess body fat (will be discussed in-depth in part 3).

In other studies, control groups lost more fat than the keto groups, but the results weren’t statistically significant (Meckling et al., 2004; Johnston et al., 2006). It’s possible these studies were statistically underpowered to detect differences.

“We propose that the women randomized to the low-fat diet systematically underreported their energy intake. Such an error could account for the apparent paradox in the estimates of energy balance in groups of subjects with significantly different amounts of weight loss. Based on the low-carbohydrate dieters’ reported energy intake at baseline, 2 months, and 4 months, the expected weight loss was 6.8 kg and 11.8 kg at 2 and 4 months, not substantially different from their actual weight loss of 6.7 kg and 9.8 kg, respectively. In contrast, the weight loss predicted from the diet records of the low-fat group was 6.5 kg and 12.4 kg at 2 and 4 months, overestimates compared with the measured results of 4.8 kg and 6.1 kg. Thus, actual weight loss approximates the expected weight loss for the low-carbohydrate group but is less than expected for the low-fat group. One reason for underreporting by the low-fat dieters may have been that they were following a prescribed energy restriction and so were faced with a limit in daily energy intake. Because we did not restrict energy intake in the low-carbohydrate group, it is plausible that they felt less pressure to meet any goals for energy intake. If we had given a prescribed energy restriction to the low-carbohydrate group, they too might have underreported their daily intake.” - Brehm et al., 2005

Ketone levels and fat loss

Posed with the uncertainty above, we wanted to further investigate how fat loss is affected by ketosis. In particular, does higher ketone levels lead to greater fat loss?

The graph below shows how much fat keto groups lost in studies where ketone levels were reported. The ketone levels shown are from the last week of the study, average ketone levels, or the ketone level that occurred most commonly during the study. Total fat mass lost was divided by the duration of the study (number of weeks), allowing us to directly compare weekly losses in different studies.

Correlation between ketones (BHB) and weekly fat loss

Note: The red line shown in the regression model is the R^2 line.Y-axis: weekly fat lossX-axis: BHB levels (mmol/l)Each dot represents a study group (keto or control) where BHB levels and fat loss were reported

However, the correlation disappeared (Pearson’s R = 0.06, R^2 = 0.00) after a clear outlier (Q1 – 1.5 × IQR or Q3 + 1.5 × IQR) was removed (the keto group in Johnstone et al., 2008) . The strength and direction of the correlation seems to rely purely on the inclusion/exclusion of this group.

As a side note, these correlations serve as an example of how one could easily “torture” data (tactical exclusions and inclusions) to make it admit to anything.

Limitations of our correlations

We've run correlations between study averages and we don’t have access to individual data points. Averaged data points aren’t as accurate as individual data.

All data here are presented as comparisons made towards baselineA green background indicates that the changes were statistically significant from baseline.* indicates that the specified outcomes was improved significantly from baseline.

Mobile friendly tables with links to studies coming soon!

Critical thinking exercise

Do changes in VO2max accurately reflect how performance improves or becomes worse?

Which confounders could influence endurance performance during the intervention?

It is also important to remember that a small experimental group could mean one person affects the overall mean (to be covered in future limitations article).

Figure by Brad Dieter and Adam Tzur, based on data from Phinney et al., 1983 were subjects maintained performance on average, yet that is not the case when looking at individual data.

It was later shown in a crossover study that two weeks of ketogenic dieting, compared to two weeks of high-carbohydrate dieting, benefited prolonged submaximal exercise (60% VO2max) without affecting high-intensity endurance performance (90% VO2max) in cyclists. However, nutrition and training data was not provided, which makes it difficult to know exactly what the participants were eating or doing (Lambert et al., 1994).

One study looked at maximal and mean power output after only three days of a ketogenic diet in untrained subjects. There was a significant decrease in mean power and a trending decrease in maximal power output which is likely due to a decrease in glycogen (Langfort et al., 1997).

In 2013, Klement et al., published a pilot case study on the impact of a self-prescribed ketogenic diet (Klement et al., 2013). Five weeks of a ketogenic diet did not impair running performance, but large inter-individual differences existed. Importantly, there was no significant difference in Vo2 max. There was no control group or training data in this study(see above).

In a well-designed study, Burke and colleagues followed race-walkers for 3 weeks over one or two competitions (Burke et al., 2017). They compared high CHO to a ketogenic diet while the athletes were in a slight caloric deficit. The main findings were a) all groups improved performance compared to baseline and b) those on a ketogenic diet failed to improve their 10km performance during the second competition. However, their exercise economy decrease.

In one of the longest ketogenic diet studies to date, Zinn et al, used aged endurance athletes (Zinn et al., 2017). They found a decrease in performance using several tests such as time to exhaustion, Vo2 max, and peak power.

The keto endurance studies are a mixed bag. Many of them have very small sample sizes and are relatively short in duration. Since there seems to be a keto adaptation period, subjects are expected to underperform after a week or two on the diet (Phinney et al., 1980). The results might give us the wrong impression that keto leads to underperforming, even though things might be different if the studies were longer.

Most studies did not control training. They either asked subjects to maintain their regular training program or do no training. This is good for ecological validity. In other words, it tells us how the diet plays out in practice when we don’t control the variables of a person’s life.

For example, if people naturally exercise less when on a keto diet because of fatigue or weakness, then a drop in physical performance would probably follow. But if the participants were “forced” to exercise during the study, then they might improve their performance. However, when people in real life decide to go on a ketogenic diet, they won’t have scientists, doctors, and personal trainers watching over them. So the first scenario is more realistic. The second scenario is necessary when trying to figure out how keto affects people who train.

Lastly, there are many ways to measure aerobic and anaerobic performance (average power, peak power, time to exhaustion, race performance, speed in timed trials, VO2max, etc.). We must remember that one test isn’t necessarily enough to make conclusions about overall “endurance performance”. The term itself is somewhat vague and depends on the sport or activity in question.

In summary, most of the studies suggest that measures of aerobic and anaerobic short-term performance are either decreased or maintained with a ketogenic diet.

How keto affects strength

Note: when calculating averages, the highest numbers were used when ranges were given.

Studies with control groups

Sawyer et al., 2013 excluded due to lack of baseline testing, no washout period, as well as reporting highly unlikely strength levels for women (mean bench press 1RM ~84 kg for men and women with a SD of 3 kg, average body weight of women were ~65 kg). Roberson et al., 2017 excluded since it is an unpublished/not-yet-reviewed abstract.

Studies without control groups (single-arm trials)

All data here are presented as comparisons made towards baseline.

A green background indicates that the overall changes were statistically significant from baseline.* indicates that this specific outcome significantly differed from baseline.

Discussing the keto strength literature

In a study using eight elite gymnasts over 30 days, Paoli et al., found no strength differences in squat jump, countermovement jump, chin-ups, or dips when comparing those on a western diet vs ketogenic diet. There was a small non-significant trend for those on a western diet in pushups (+7) and a small non-significant trend in the leg closed barrier test (+2) for subjects on a keto diet. A major limitation of the study is protein wasn’t matched during the diets and there was no training data.

Another study was completed in a group of taekwondo athletes using a ketogenic diet vs control (Rhyu and Cho, 2014). Rhyu and colleagues found no difference in grip strength.

CrossFit contains a form of resistance training and one keto trial was run for 6 weeks in trained subjects(Gregory et al., 2017). Interestingly, after 6 weeks there was no difference in long jump or vertical jump between keto and control group. All subjects in the study improved total CrossFit performance time and overall power. A few limitations for this study include lack of sufficient ketone measurements, food reporting, and the keto group eating 200 less calories + 10 grams more protein per day.

Lastly, a RCT in untrained older men (age = 59) compared a low-fat diet vs a ketogenic diet, with or without exercise, over the course of 12 weeks (Wood et al., 2012). Comparing across groups that trained, the low fat diet group increased chest press strength twice as much as the keto group. There was also a difference favoring the low-fat diet in leg press (+26%). In contrast, a study by Merielles et al., found no difference in strength measurements when comparing a keto group to a control group (Meirelles and Gomes, 2016).

There are few resistance training studies that use a ketogenic diet. Most of the studies find that both the keto group and the control group can increase their strength.

Overall, we need more protein and calorie-matched studies that also measure ketone bodies. With these limitations noted, keto groups did maintain or increase their strength in the studies. Though, typically not to the same extent as the control group. This is promising because it suggests that a ketogenic diet doesn’t have to entail a loss of strength, but it also implies that people looking to maximize their strength should be careful about choosing keto.

Individual diet responses

Most of our data here deals with how people respond to a diet, on average. However, some studies have also reported the level of success for every individual.

Below you see figures from different studies. Every dot, circle, X, or line represents an individual. You can see that some individuals lost very little weight, while some lost a lot.

Whenever a mean is reported, these individual data points are not included. However, standard deviations (a measure of spread) can be reported alongside the mean to help give us an idea how much variation there is in individual responses.

Exclusion criteria

General exclusion criteria:

All animal studies

Studies with major methodological issues

Studies that do not measure FM/FFM

Studies that lack of data reporting, to the point where estimating changes in body comp would involve a lot of guesswork and assumptions (i.e. data only presented as percentages, or in low-resolution figures, making estimations difficult)

Our conflicts of interest

This article was written and published without sponsorship from any company, individual, or organization. This website, SCI-FIT, receives no compensation or funding from any source, and is not affiliated with any other companies or organizations.

Adam Tzur is the head of SCI-FIT. No COIs to report.

Brandon Roberts works at the University of Alabama at Birmingham. He is also employed by The Strength Guys. No COIs to report.